1. Field of the Invention
This invention relates to laser shock peening and, more particularly, to methods for laser shock peening a single side of an article.
2. Description of Related Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of an article. Laser shock peening typically uses one or more radiation pulses from high power pulsed lasers to produce an intense shock wave at the surface of an article similar to methods disclosed in U.S. Pat. No. 3,850,698 entitled xe2x80x9cAltering Material Propertiesxe2x80x9d; U.S. Pat. No. 4,401,477 entitled xe2x80x9cLaser Shock Processingxe2x80x9d; and U.S. Pat. No. 5,131,957 entitled xe2x80x9cMaterial Propertiesxe2x80x9d. Laser shock peening, as understood in the art and as used herein, means utilizing a pulsed laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force at the impingement point of the laser beam by an instantaneous ablation or vaporization of a thin layer of that surface or of a coating (such as tape or paint) on that surface which forms a plasma.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following U.S. Pat. No.: 5,756,965 entitled xe2x80x9cOn The Fly Laser Shock Peeningxe2x80x9d; U.S. Pat. No. 5,591,009 entitled xe2x80x9cLaser shock peened gas turbine engine fan blade edgesxe2x80x9d; U.S. Pat. No. 5,531,570 entitled xe2x80x9cDistortion control for laser shock peened gas turbine engine compressor blade edgesxe2x80x9d; U.S. Pat. No. 5,492,447 entitled xe2x80x9cLaser shock peened rotor components for turbomachineryxe2x80x9d; U.S. Pat. No. 5,674,329 entitled xe2x80x9cAdhesive tape covered laser shock peeningxe2x80x9d; and U.S. Pat. No. 5,674,328 entitled xe2x80x9cDry tape covered laser shock peeningxe2x80x9d, all of which are assigned to the present Assignee.
Laser peening has been utilized to create a compressively stressed protective layer at the outer surface of an article which is known to considerably increase the resistance of the article to fatigue failure as disclosed in U.S. Pat. No. 4,937,421 entitled xe2x80x9cLaser Peening System and Methodxe2x80x9d. These methods typically employ a curtain of water flowed over the article or some other method to provide a plasma confining medium. This medium enables the plasma to rapidly achieve shock wave pressures that produce the plastic deformation and associated residual stress patterns that constitute the LSP effect. The curtain of water provides a confining medium, to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP""D, to create the beneficial compressive residual stresses.
The pressure pulse from the rapidly expanding plasma imparts a traveling shock wave into the component. This compressive shock wave caused by the laser pulse results in deep plastic compressive strains in the component. These plastic strains produce residual stresses consistent with the elastic modulus of the material. Dual sided simultaneous laser shock peening includes simultaneously striking both sides of an article by two laser beams in order to increase the compressive residual stress in the material. The laser beams are typically balanced in order to minimize material distortion. There are some applications for single sided laser shock peening. The initial compressive waves pass through the material from each of the sides and are reflected back from the interface of the two initial compressive waves. The reflected waves turn into a tension wave. The reflected tension waves from both sides can meet at a mid-plane in the same axial direction and reinforce each other leading to a high level of stress at the mid-plane.
There are some applications like airfoil leading edges of blisks where only one side of the article is easily accessible with a laser beam. A single sided LSP processing would be very useful but the compressive shock (stress) wave traveling through the metallic article is reflected from the other side of the article and returns as a tensile stress wave. The reversal of the stress from compressive to tensile is caused by the lower shock impedance of the adjoining material (usually room air). The returning tensile stress wave tends to undo at least a portion of the beneficial effects of the original compressive wave, i.e. lowering the amount of compressive residual stress imparted by the laser shock peening.
Thus, it is highly desirable to have a single sided laser shock peening process that avoids reduction or loss of effectiveness of the beneficial compressive strains from laser shock peening caused by reflected tensile waves.
A method for single sided laser shock peening an article includes laser shock peening a laser shock peening surface on a first side of the article while maintaining an opposite second surface on a back side of the article in acoustic communication with a shock attenuating material. The second surface is opposite the laser shock peening surface. The shock attenuating material is a material that does not allow tensile waves to be reflected back off the back side through the article. The shock attenuating material is a material that has a shock impedance equal or higher than that of the article.
The shock attenuating material may be a liquid metal and the article made from a titanium alloy. One such article is a gas turbine engine airfoil and the surfaces may be on an edge of the airfoil. A particular embodiment of the invention includes single sided laser shock peening a leading edge of the airfoil. The airfoil may be part of an integrally bladed disk. One liquid metal is mercury.
Another shock attenuating material is a solid attenuating material and a liquid metal interface, such as mercury, may be disposed between the article and the solid attenuating material. The shock attenuating material may be one that dissipates compressive waves caused by the laser shock peening. Another type of the shock attenuating material reflects back compressive shock waves caused by the laser shock peening through the back side of the article.
The liquid shock attenuating material or liquid metal interface may also be a slurry formed by mixing a suitable amount of metallic particles with a carrier liquid to achieve the desired shock impedance. Examples of such metallic particles are copper, brass or tungsten and one example of a suitable liquid carrier is a non-corrosive lubricant.